Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require hundreds of connections to external devices.
Semiconductor chips have commonly been connected to electrical traces on mounting substrates using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on the microelectronic assembly, and fatigue of the connections due to changes in the sizes of the chip and the substrate under thermal expansion and contraction.
Structures that have been used to successfully address the foregoing problems are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; and 5,455,390. Structures according to certain of the embodiments taught in those patents comprise a flexible, sheet-like element having a plurality of terminals disposed thereon. Flexible leads are used to connect the terminals with contacts on a first microelectronic element such as an integrated circuit. The terminals may then be used to test the microelectronic chip, and may be subsequently bonded to a second microelectronic element. The flexible leads permit thermal expansion of various components without inducing stresses in the connection.
Commonly assigned U.S. Pat. No. 5,518,964 ("the '964 patent"), hereby incorporated in its entirety herein, discloses further improvements in microelectronic connections. In certain embodiments of the '964 patent, a flexible, sheet-like element has a first surface with a plurality of elongated, flexible leads extending from a terminal end attached to the sheet-like element to a tip end offset from the terminal end in a preselected, first horizontal direction parallel to the sheet-like element. The tip ends have bond pads for connection to a microelectronic element. As the term is used herein, "microelectronic element" encompasses circuit boards, integrated circuits, connection components such as polyimide or other dielectric sheets, and other components used in microelectronic circuitry. Each of the plurality of leads is simultaneously formed by moving all of the tip ends of the leads relative to the terminal ends thereof so as to bend the tip ends away from the sheet-like element. This is accomplished by relative movement between the sheet-like element and the microelectronic element.
The tip ends of the leads are initially attached to the sheet-like element. The initial position of the bond pad on the tip ends is thereby fixed with respect to the terminal ends in order to facilitate attachment to the microelectronic element.
Various lead configurations are disclosed in the '964 patent. In one such configuration, the leads comprise straight, elongated bodies of conductive material extending between terminal ends connected to a dielectric sheet-like element and tip ends to be connected to a microelectronic element. The terminal end of the lead is attached through a via in the sheet-like element to another microelectronic element on the other side of the sheet-like element. The attachment of the tip ends of the leads to the sheet-like element is releasable. After bonding the tip ends to the microelectronic element, the leads are formed in their final configuration by moving the sheet-like element and the microelectronic elements relative to each other in two directions: in a vertical direction away from each other, and in a horizontal direction parallel to the sheet-like element. As a result, the tip end of the lead is separated from the sheet-like element and traces an arcuate path relative to the other end of the lead. That movement prevents stretching of the lead during formation and results in an S-shaped configuration of the lead that is advantageous in absorbing further relative movement between the sheet-like element and the microelectronic element due to thermal expansion/contraction during operation of the resulting device.
In another lead configuration taught in the '964 patent, the lead is initially a curved strip disposed on a surface of the sheet-like element. A terminal end of the lead is connected to a terminal through a via in the sheet-like element and a tip end is bonded to a microelectronic element. In forming those leads to a final configuration, the sheet-like element and the microelectronic element are moved away from each other in a vertical direction only. The curve of the lead is partially straightened by the relative movement of the elements. The "slack" created by the initial curve in the lead permits vertical displacement of the microelectronic components without the necessity of providing additional lead length by horizontally displacing the components.
A number of such configurations are disclosed in the '964 patent. An S-shaped lead structure having two small, opposite bends in each lead permits nesting of adjacent leads in configurations requiring a high lead density. A U-shaped lead configuration permits larger relative displacement of the microelectronic components in a vertical direction without a corresponding horizontal displacement. Other such configurations are also disclosed.
Commonly assigned U.S. patent application Ser. No. 08/712,855, filed Sep. 12, 1996, which is incorporated by reference in its entirety herein, discloses additional lead configurations. One lead disclosed in that application comprises a body section and two flexible leg sections separated by a single slot. The ends of the leg sections comprise tip and terminal ends. The slot has a cutout at its end to reduce stress concentration.
There is presently a need for a curved lead configuration that performs several important functions. First, the lead should be capable of extending a sufficient vertical distance out of the original plane of the lead in order to allow relative movement of the two microelectronic components. While larger leads inherently extend further than smaller leads, the pitch of the lead array should also be small enough to match the pitch of the contacts of the microelectronic component. It is therefore desirable that the extension as a percent of the pitch of a corresponding grid array be maximized.
In some instances, stress at a given point in the lead may be increased by concentrated torsion or bending forces resulting from the particular lead geometry. The stress undergone by the lead during extension should therefore be minimized by providing rounded corners where possible and by minimizing strain caused by twisting and bending during extension. A large, concentrated stress in the lead during extension could result in failure of the lead in service by exceeding the ultimate tensile strength of the material or by exceeding the fatigue limit.
The lead should also be simple in geometry. While current photolithographic techniques permit relatively high resolution, certain complex shapes at extremely small pitches may be beyond the capability of present manufacturing processes. If possible, a curved lead design should therefore incorporate a minimum of geometric features, and the function of the lead should not rely on fine detail.
The lead may be releasably attached to the lead bearing surface for substantially its entire length, as described in commonly assigned U.S. patent application Ser. No. 08/547,170 filed Oct. 24, 1995, which is hereby incorporated by reference in its entirety herein. The peel stress undergone by such a lead during separation of the microelectronic elements from the lead-bearing surface should be minimized. As the lead is pulled from the surface, a tensile force is exerted on the portions of the lead already freed from the surface. Peel stress may be excessively high when a portion of a lead having a relatively small cross-section must be subjected to tension sufficient to peel a relatively large area of the lead from the surface.
Finally, any lead configuration has some amount of self-inductance, limiting the speed with which that lead can reliably convey an electrical signal. Certain lead configurations, however, may limit that inherent inductance through geometries that create opposing or canceling magnetic fields, thereby reducing resistance to changes in current flow. For example, it is believed that parallel conductors in close proximity tend to cancel corresponding magnetic fields, thus reducing the self-inductance of the overall lead.
Still further improvements in the above-described lead configurations would be desirable.